A common source of environmental noise is sound produced by engines which is transmitted to the environment by means of air or other gas moving through a conduit. For example, fans used for moving air through air ducts in building HVAC systems generate a great deal of low frequency noise. Although a fan can be isolated from the building's inhabitants, the air duct through which it forces air provides an acoustic pathway for the transmission of noise. Similarly, the noise produced by internal combustion engines necessarily escapes to the environment through the engine's exhaust pipe. The first and still most commonly used method for reducing these types of environmental noise is the interposition of a muffler in the duct or exhaust pipe. Mufflers either cause some of the noise to be reflected back to the source by means of an acoustic impedance discontinuity or cause some of the noise to be absorbed and thus converted into heat by a sound absorptive lining.
Although such mufflers are generally effective in reducing noise, they also necessarily introduce a flow restriction in the duct or exhaust pipe. A flow restriction in an air duct obviously diminishes the efficiency, at which a fan can deliver air through the duct. A flow restriction in the exhaust pipe of an internal combustion engine means that the engine must exhaust against a higher pressure which reduces the fuel efficiency of the engine.
In the case of air ducts which transmit noise from machinery such as HVAC fans or industrial blowers, the reduction in efficiency caused by mufflers has provided part of the impetus for the development of so-called "active" noise reduction systems. In contrast to "passive" noise control techniques (i.e., mufflers), active systems control the propagation of sound in a duct by generating additional sound waves within the duct having the same amplitude but with opposite phase to those emanating from the noise source. The additional sound waves thus produce an effective acoustical short circuit to cancel the unwanted noise. Systems for such active noise cancellation generally comprise a loudspeaker mounted so as to radiate sound into the duct, an input microphone for sensing sound upstream (i.e., toward the noise source) from the loudspeaker, an error microphone for sensing sound downstream from the loudspeaker, and a controller for driving the loudspeaker in accordance with signals received from the input and error microphones which attempts to minimize the sound radiated into the external environment.
Such active noise control systems as described above have been successfully used for attenuating sound in air ducts. Similar systems can be employed in the case of an internal combustion engine. Mounting a loudspeaker directly within an exhaust pipe, however, is impractical due to the high temperature of the exhaust gases. One prior attempt to solve this problem has involved mounting the loudspeaker within an outer plenum chamber which concentrically surrounds the exhaust pipe. The output of the loudspeaker then combines with the engine noise where the latter exits the exhaust pipe to effect the desired noise cancellation. The error microphone can be mounted near the exhaust pipe outlet as well for providing the error feedback signal. A feedforward signal can be derived from either an electrical-signal proportional to engine speed or from an input microphone designed for a high temperature environment mounted within the exhaust pipe.
The use of a noise cancellation loudspeaker mounted within an outer plenum chamber, however, does not allow for very effective cancellation of the sound exiting the exhaust pipe since sound from the loudspeaker and from the engine cannot be directly combined. A much more effective noise cancellation effect could be obtained if the loudspeaker's sound were to radiate directly into the engine exhaust pipe.